The invention relates to semiconductor wafer testing and more particularly to characterizing the thickness and capacitance in the presence of substantial leakage current of a dielectric layer on a semiconductor wafer.
As is known in the art, semiconductor devices often contain dielectric layers (e.g., silicon dioxide) grown/or deposited on a semiconductor substrate (e.g., silicon). Semiconductor wafers including dielectric layers are used in manufacturing microelectronic devices such as metal-oxide-semiconductor (MOS) capacitors, MOS-field effect transistors (MOSFET), and related integrated circuits.
During manufacture of microelectronic devices, the thickness of the dielectric layer must be frequently monitored with high precision. Typically, the dielectric thickness is measured by optical or electrical methods. Optical ellipsometric methods for determining a dielectric thickness are described for example in U.S. Pat. No. 5,343,293 and the references therein. An electrical method for determining the thickness of a dielectric utilizes capacitance measurements of MOS capacitors fabricated, for the test purposes, on the dielectric layer. Once measured, the capacitance can be used to calculate an xe2x80x9cequivalent oxide thicknessxe2x80x9d (EOT), i.e. the thickness of a SiO2 layer that would produce the same measured capacitance. In other electrical methods, the dielectric thickness can be determined without fabricating MOS test capacitors by charging the surface of the dielectric layer with a corona discharge and measuring the resulting surface potential of the charged dielectric layer with a Kelvin or a Monroe probe. These techniques are discussed, for example, in U.S. Pat. Nos. 5,485,091, to R. L. Verkuil, and 6,037,797, to J. Lagowski et.al. In these methods a known electric charge xcex94QC is placed on the surface of a dielectric layer (for example, on the surface of a SiO2 layer on a silicon wafer) by a precisely calibrated corona discharge source. The dielectric layer thickness can be determined from the value of xcex94QC /xcex94V where xcex94V is the change of the dielectric surface potential caused by the charge xcex94Qc.
Due to substantial leakage of current across the dielectric layer, via tunneling, the methods described in U.S. Pat. Nos. 5,485,091 and 6,037,797 are ineffective for determining the thickness of ultra-thin dielectrics films, i.e. dielectric films having an EOT equal to or less than about 40 xc3x85. When leakage current tunnels through the dielectric layer, the charge xcex94QC is reduced by the amount of charge transported across a dielectric layer during the time of charging and measuring xcex94V. In thicker dielectric layers, such as SiO2, the leakage current is typically below 10xe2x88x9212 A/cm2 for a corona charge, xcex94QC, of about 2xc3x9710xe2x88x927 C/cm2. During a typical measuring time for these techniques, e.g., 100 seconds, the leakage of current via tunneling through the thicker dielectric layer reduces xcex94QC by about 10xe2x88x9210 C/cm2. The value 10xe2x88x9210 C/cm2, however, is practically insignificant relative to xcex94QC, i.e., 2xc3x9710xe2x88x927 C/cm2. For ultra-thin dielectric layers, the leakage currents are orders of magnitude larger than the leakage currents for thicker dielectric layers, e.g., often exceeding 10xe2x88x929 A/cm2. As a result, leakage of current over a measuring time of 100 seconds will reduce the value of xcex94QC by about 10xe2x88x927 C/cm2 and will cause significant errors in calculating the thickness of the dielectric layer. Additionally, in high accuracy measurements on thin dielectric layers, the value of xcex94V is typically corrected to account for a voltage drop, xcex94VSB, across the semiconductor surface space charge layer by replacing xcex94V with the expression xcex94V-xcex94VSB. Errors in determining the dielectric layer thickness are more severe in this instance because leakage current not only corrupts the value of xcex94QC but also the calculation of xcex94VSB.
In general, the invention relates to an apparatus and method for producing non-contact electrical measurements of capacitance and thickness of ultra-thin dielectric layers on semiconductor substrates (wafers). The apparatus and method produces effective and accurate measurements of the dielectric layer thickness despite substantial leakage of current across the layer and no apriori knowledge of the relationship between the leakage current characteristics, i.e., measured electrical properties such as voltage and current, and thickness of the dielectric layer. Ultra-thin dielectric layers have an equivalent oxide thickness equal to or less than about 40 xc3x85. As used herein, the term xe2x80x9cdielectricxe2x80x9d includes but is not limited to oxides, e.g., SiO2, Ta2O5, Al2O3, nitrides, e.g. Si3N4, and barium strontium titianate (BST). The non-contact electrical technique can be used to record multiple, repeatable measurements of ultra-thin dielectric capacitance and thickness at the same location on the wafer under highly reproducible conditions.
In an aspect, the method of determining the thickness of a dielectric layer on a semiconductor wafer includes depositing an electric charge sufficient to cause a steady state condition in which charge current is equal to the leakage current; measuring the potential of the dielectric surface; and comparing the measured parameters to calibrated parameters to derive the dielectric layer thickness.
In another aspect, the method of determining the thickness of a dielectric layer deposited on a semiconducting wafer includes depositing an ionic charge onto a surface of the dielectric layer deposited on the semiconducting wafer with an ionic current sufficient to cause a steady state condition; measuring, via a non-contact probe, a voltage decay on the dielectric surface as a function of time; and determining the thickness of the dielectric layer based upon the measured voltage decay. The method can further include measuring the voltage decay after terminating the deposition of ionic charge.
In another aspect, the method of determining the thickness of a dielectric layer deposited on a semiconducting wafer includes depositing an ionic charge onto a surface of the dielectric layer with an ionic current sufficient to cause a steady state condition; ceasing ionic charging after establishing the steady state condition; measuring, via a non-contact probe, a voltage decay on the semiconducting wafer as a function of time after ceasing the ionic charging; analyzing the voltage decay to determine a characteristic of the measured voltage decay, the characteristic of the measured voltage decay being selected from the group consisting of an initial surface potential, V0, a surface potential at a time greater than t=0, VD, and an initial rate of voltage decay, dV/dt|t=0; and determining the thickness of the dielectric layer based upon the characteristic of the measured voltage decay.
Embodiments of the invention can include one or more the following. The dielectric layer has a thickness of about 40 xc3x85 or less. The steady state condition results when the ionic current equals a leakage current flowing from the semiconducting wafer and across the dielectric layer. The step of determining the thickness of the dielectric layer includes determining the initial surface potential, V0, on the dielectric layer from the measured voltage decay. The initial surface potential, V0, is determined by extrapolating the measured voltage decay back to t=0. The step of determining the thickness of the dielectric layer further includes using the initial surface potential, V0, in a linear expression to calculate an equivalent oxide thickness, T, of the dielectric layer, the linear expression given by the relationship V0=aT+b. The coefficients a and b in the linear expression are determined by a calibrating procedure. The calibrating procedure comprises recording a decay voltage on a plurality of semiconducting wafers each having a known dielectric layer thickness, and determining from each measured voltage decay an initial surface potential. The semiconductor wafer is p-type silicon having a doping of about 1xc3x971015 cm3, the dielectric layer is SiO2, the corona charge has positive polarity, the thickness of the dielectric layer is about 40 xc3x85 or less, a is about 88 mV per xc3x85, and b is about xe2x88x92550 mV. The method further includes rescaling the coefficient b by adding the value xcex94b, where xcex94b[mV]=xe2x88x9226 ln(NA2/NA1) in which NA1 is a dopant concentration in a calibrating semiconducting wafer having a known dielectric layer thickness and NA2 is a dopant concentration in the semiconducting wafer being measured. The step of determining the thickness of the dielectric layer includes determining the surface potential at a time greater than t=0 on the dielectric surface from the measured voltage decay. The surface potential is determined at a time of about 1 second after t=0. The step of determining the thickness of the dielectric layer includes using the surface potential at a time greater than t=0, VD, to calculate a dielectric thickness, T, via the expression VD=cT+d, in which the coefficients c and d are derived from a calibrating procedure. The method further includes the step of determining the thickness of the dielectric layer includes using the surface potential at a first time greater than t=0, VD1, and a second time greater than t=0 and different than the first time, VD2, to calculate a dielectric thickness, T, via the expression VD1xe2x88x92VD2=c1T+d1, in which the coefficients c1 and d1 are derived from a calibrating procedure. The calibrating procedure includes measuring a voltage decay on a plurality of semiconducting wafers each having a known dielectric layer thickness, and determining from each measured voltage decay a surface potential, VD, at the same time in the decay, the time being greater than t=0. The steps of depositing a charge onto a surface of the dielectric layer, measuring the voltage, V0, and determining the thickness of the dielectric layer all occur in about 7 seconds or less. The method further includes determining the capacitance of the dielectric layer deposited on the semiconducting wafer. The capacitance is obtained from the relationship Cox=JC/R, where JC the ionic current at the steady state condition, R is the initial voltage decay rate, dV/dt|t=0, derived from the measured voltage decay. The voltage decay is measured after terminating the deposition of ionic charge. The steps of depositing ionic charge, measuring the voltage decay, and determining the thickness are performed on a measurement area smaller than a total surface area of the semiconducting wafer. The method further includes depositing a precharging ionic charge on the dielectric layer. The method further includes illuminating the dielectric surface to eliminate the semiconductor surface depletion layer and its contribution to V0, VD, and dV/dt|t=0. The method further includes performing the steps of depositing ionic charge, measuring voltage decay, and determining the dielectric thickness on a plurality of measurement sites on the dielectric layer.
In ultra-thin dielectric layers, the leakage current mechanism may differ from the mechanism of leakage in thicker dielectrics layers. For example, Fowler-Nordheim (F-N) tunneling dominates in thick SiO2 layers that exceed 40 xc3x85. In this process, electrons tunnel over a distance of about 40 xc3x85 from the Silicon to the SiO2 conduction band and then travel across the rest of SiO2 layer. For SiO2 layers thinner than 40 xc3x85, electrons tunnel directly across the entire dielectric layer, producing much larger currents for lower oxide voltages. For non-SiO2 dielectrics, leakage current mechanisms, other than F-N tunneling or direct tunneling, such as Schottky emission or Frenkel-Poole emission can be dominant. (For discussion of conduction processes in insulators, see for example, xe2x80x9cPhysics of Semiconductor Devicesxe2x80x9d by S. M. Sze, John Wiley and Sons, 1981; p. 402 to 407). As a result, the exact tunneling current equation relating the layer thickness to electrical parameters such as the tunneling leakage current and the voltage drop across a dielectric layer may not be known apriori. Advantageously the method of this invention, unlike the method discussed in U.S. Ser. No. 09/451,652, can be used to determine the thickness of a dielectric layer without knowing apriori the exact tunneling current equation relating the measured electrical parameters to the dielectric layer thickness. The apparatus and methodology can be used to determine dielectric layer thicknesses with a 0.01 xc3x85 sensitivity, a factor of 10 better than standard methods, a repeatability of about 0.03 xc3x85 for a 30 xc3x85 thick dielectric layer and about 0.04 xc3x85 for a 17.6 xc3x85 thick dielectric layer, and a tolerance to changes in the corona ionic current of about a factor of 20 greater than standard methods.
The details of one or more embodiments of the invention are set forth in the accompanying drawings and the description below. Other features, objects, and advantages of the invention will be apparent from the description and drawings, and from the claims.